X COMM: Communication check
X COMM followed by a number means input is inhibited because of a communication check. The link between the emulator and the host isn't working or isn't ready. For TN3270 sessions in IBM's emulators, such as Host On-Demand, the number (654, 657 or 659, for example) tells you which step of the connection failed.
Other names for this code
Also written as X COMM, COMM 659, COMM 654, COMM 657, COMM 658, COMM 655, COMM 662, COMM 666, 3270 communication check, COMM check TN3270, lightning bolt 3270 status line.
What it means
A communication check means the problem is in the connection, not in the application or anything you typed. On the status line it shows as the word COMM, or as a lightning bolt symbol when the session uses a 3270 font, followed by a three-digit number. The keyboard is locked while it shows, because there is nothing to send your input to.
For TN3270 and TN3270E sessions, IBM's emulator documentation lists these codes:
- 654: the TN3270E server refused the LU name. It may be in use, not defined, the wrong LU type, or the negotiation failed.
- 655: the socket is connected and waiting for Telnet negotiation to finish. It also appears when SSL is off but the port expects SSL.
- 657: the TCP/IP connection is being set up. With SSL it can mean the port isn't an SSL port or a signer certificate has expired.
- 658: the TCP/IP connection for TN3270E is initializing.
- 659: the TCP connection failed or was dropped by either side.
- 662: the server's certificate isn't trusted.
- 663: the name in the server's certificate doesn't match the server.
- 664: the secure connection couldn't be completed.
- 665: the server's certificate isn't valid yet.
- 666: the server's certificate has expired.
Common causes
- The host name, IP address or port in the session profile is wrong, or the TN3270 server isn't listening on that port (usually 657 or 659).
- The session asks for a specific LU name that another session already uses or that isn't defined on the server (654).
- SSL settings don't match the port. The session uses SSL against a plain port, or plain Telnet against an SSL port (655 or 657).
- A firewall, VPN or idle timeout dropped the connection, or the server closed it (659).
- The server's certificate isn't trusted on this PC, has expired, or has a different name from the one you connect to (662 to 666).
How to find the problem
- Read the number after COMM. It tells you which stage failed, and the list on this page gives the meaning.
- Check the session's host name, port, LU name and security settings against what the mainframe team gave you.
- Ask whether others connecting to the same server have the same code. If they do, the server or network side is down.
- For certificate codes, view the server certificate and check its dates, issuer and host name.
- If the code doesn't explain itself, turn on the emulator's communication trace and reproduce the problem. IBM suggests a transport-level trace for 654.
How to fix it
- For 657 and 659, check the address and port, check that you can reach the host, and reconnect. If it drops again after idle time, ask for keepalive settings or a longer timeout.
- For 654, use a different LU name or a pool name, or ask the network team to free or define the LU.
- For 655, match the SSL setting to the port. Most sites use one port for TLS and another for plain Telnet.
- For 662 to 666, install the right signer certificate, connect with the name in the certificate, or ask the host team to renew an expired certificate.
- After the connection is back, sign on again. Any screen you sent just before the check may not have reached the application.
Read this error in WND Terminal
For a visible X COMM message, Explain this screen can help interpret the screen under IT’s configured AI policy. It does not establish the cause of a rejected host data stream. Preserve the exact status and follow the diagnostic steps in this article.
Try the WND Terminal sample with fictional data · 3270 emulator features and installation · 5250 emulator for IBM i
IBM documentation
IBM Personal Communications documentation. Use the documentation for your installed release.
This link opens the relevant IBM reference library or lookup instructions. Match the complete code and your installed product release; it is not a code-specific review.
Related codes
Questions
What does COMM 659 mean?
The Telnet TCP connection to the TN3270 server failed or was dropped. Check the address and port, then reconnect, and look for a firewall or idle timeout if it keeps happening.
Why does COMM 657 stay on the screen?
The emulator is still trying to set up the TCP/IP connection. If it never completes, the server may be unreachable, or an SSL session is pointed at a port that doesn't use SSL.
Will pressing Reset clear a COMM check?
No. The lock goes away only when the connection is working again.